The present invention is directed to a tool bit holder or chuck for hand-held tools with the tool bit holder having two rotary entrainment or driving grooves open at a free end of the shank with the grooves disposed diametrically opposite one another. The tool bit holder has shaped projecting rotary entrainment means engageable in the rotary entrainment grooves for driving the tool bit shank. Further, the tool bit shank has laterally closed recesses disposed diametrically opposite one another and the tool bit holder contains radially displaceable locking members engageable within the recesses.
A tool bit and tool bit holder of the type described above is disclosed in DE-OS 25 51 125. In this known tool bit holder the rotary entrainment or driving means engageable in the rotary entrainment grooves effect transmission of the torque, while the locking members, radially displaceable within the tool bit holder and engageable into recesses in the tool bit shank, are laterally closed for axial retention of the tool bit. The radial displacability of the locking members is realized by an actuation sleeve which can be turned or displaced axially so that recesses can be brought into alignment with the locking members, whereby the locking members move radially outwardly into the recesses and release the tool bit for removal.
While the axial locking action is not intended to satisfy stringent requirements as far as loading or stress is concerned, considerable forces, due to the torque transmitted, act on the rotary entrainment means of the tool bit holder. As a result, rotary entrainment or driving means of insufficient dimensions are incapable of transmitting torques of a high magnitude, while rotary entrainment means of larger dimensions due to the larger rotary entrainment grooves required, result in a weakening of the tool bit shank with the possibility that fractures may develop in the shank. Up to the present time, an optimum compromise has not been found for symmetrical arrangements where similarly dimensioned rotary entrainment grooves, located diametrically opposite one another, are arranged to receive correspondingly dimensioned rotary entrainment means.